1. Field of the Invention
The present invention generally relates to a position control apparatus for controlling a position of a device such as an optical pickup to be used for recording or reproducing a recording disk, for example, a CD (Compact Disk), an LD (Laser Disk) or the like.
2. Description of the Related Art
An optical recording system for recording information onto an optical recording disk such as a CD-R (Compact Disk-Recordable) or the like and an optical reproducing system for reproducing information recorded on an optical recording disk such as a CD, LD or the like are required to have various servo control mechanisms in order to accurately record the information onto the optical disk or to accurately read the information from the optical disk.
Here, as one example of the various servo control mechanisms, a servo control mechanism for controlling a position of an optical pickup in a CD reproducing system, which is the apparatus to reproduce a CD, will be explained.
The CD reproducing system has a spindle motor for controlling a rotation of the CD, a pickup for irradiating a light beam to a surface of the CD and detecting a reflective light from the surface of the CD in order to read information recorded on the CD, a signal processor for decoding the information read by the pickup in order to reproduce this information, a tracking servo motor for controlling a position of an objective lens disposed in the pickup, a carriage servo motor for controlling a position of the pickup, a detecting device for detecting a quantity of a shift (displacement) between a track position and a light spot position (irradiating position of the pickup) and generating a tracking error signal, whose amplitude changes in proportion to the aforementioned quantity of the shift, and a servo control driver for generating driving signals on the basis of the tracking error signal and applying these driving signals to the tracking servo motor and the carriage servo motor in order to drive and control these motors.
In operation, the spindle motor rotates the CD. Then, the pickup irradiates the light beam to a surface of the CD and detects the reflective light from the surface of the CD. In this manner, the pickup reads the information recorded on the CD and outputs an information signal corresponding to the read information to the signal processor and the detecting device respectively. Then, the signal processor decodes this information signal, so that the information signal is reproduced.
At this time, the detecting device detects the quantity of the shift between the track position and the light spot position on the basis of the information signal outputted from the pickup, and the detecting device generates a tracking error signal on the basis of the detected quantity of the shift. Here, a three beam method or a heterodyne method is used in the detection of the quantity of the shift. Then, the detecting device outputs this tracking error signal to the servo control driver.
The servo control driver is connected with each of the tracking servo motor and the carriage servo motor. The servo control driver generates driving signals on the basis of the tracking error signal outputted from the detecting device in order to drive and control each of these motors, and the servo control driver respectively outputs these driving signals to these motors. Thus, these motors is independently driven and controlled by these driving signals.
The tracking servo motor controls the position of the objective lens disposed in the pickup on the basis of the driving signal outputted from the servo control driver. Therefore, the objective lens is moved in the radial direction of the CD by the tracking servo motor. More specifically, the light beam irradiated toward the CD from the pickup is condensed by the objective lens, and thus, a light spot is formed on the surface of the CD. Therefore, the light spot position is adjusted by the movement of the objective lens. Thus, the tracking servo motor controls the position of the objective lens such that the quantity of the shift between the track position and the light spot position is reduced in order to accurately irradiate the light beam onto each track formed on the surface of the CD through the objective lens and normally read the information recorded on the CD.
The carriage servo motor controls the position of the pickup on the basis of the driving signal outputted from the servo control driver. Therefore, the pickup is moved in the radial direction of the CD by the carriage servo motor. More specifically, the light spot position is adjusted by not only the position of the objective lens but also the position of the pickup itself. Namely, the light spot position is roughly adjusted by the movement of the pickup, and it is finely adjusted by the movement of the objective lens. Thus, the carriage servo motor controls the position of the pickup such that the light spot position is moved along the tracks arranged in spiral or coaxial on the surface of the CD in order to accurately irradiate the light beam onto each track and normally read the information recorded on each track.
In this manner, the tracking servo motor and the carriage servo motor are driven on the basis of the driving signal generated on the basis of the tracking error signal, and the tracking servo motor and the carriage servo motor control the position of the objective lens and the pickup. Therefore, the quantity of the shift between the track position and the light spot position can be reduced, and the light spot position can be moved along the track arranged in spiral or coaxial shape on the surface of the CD.
However, an eccentricity of the CD may happen in cause of an eccentricity of the tracks formed on the CD in advance or an eccentricity of the shaft of the spindle motor. Further, the eccentricity of the CD may happen when it is bad to mount the CD into a turn-table of the CD reproducing system. The eccentricity of the CD generates an eccentric noise in the tracking error signal outputted from the detection device, as shown in FIG. 9.
FIG. 9 shows the tracking error signal TE outputted from the detecting device, a converted signal CS, which is converted from the tracking error signal TE in the servo control driver, and a pulse signal PS generated from the converted signal CS in the servo control driver. The pulse signal PS is outputted to the carriage servo motor as the driving signal to drive the carriage servo motor. In FIG. 9, a high frequency sine curve is mixed in the tracking error signal TE. This high frequency sine curve is an eccentric noise generated in cause of the eccentricity of the CD. More specifically, if the eccentricity of the CD is happened, the actual track position is shifted out of the normal track position. In this case, the pickup and objective lens are respectively moved along the actual track position regardless the actual track position is shifted out of the normal track position. Therefore, when the detecting device generates the tracking error signal on the basis of the information signal outputted from the pickup, the high frequency sine curve is mixed into the tracking error signal as eccentric noise, as shown in FIG. 9.
Further, this eccentric noise is also mixed in the converted signal CS and the pulse signal PS. As a result, the carriage servo motor is controlled by the pulse signal PS mixed with the eccentric noise, and thus, the rotation of the carriage servo motor becomes unstable. Namely, the electric power in order to drive the carriage motor is obtained by the integration value of the pulse signal PS. Therefore, if the eccentric noise is mixed in the pulse signal PS, the integration value of the pulse signal PS is irregularly changed, so that the stable driving current cannot be applied to the carriage servo motor.
Furthermore, as shown in FIG. 9, the pulse signal PS is irregularly divided into the first pulse portion PP1 and the second pulse portion PP2. Therefore, if such irregularly divided pulse signals are applied to the carriage servo motor, the rotation of the carriage servo motor is unstable.
On the other hand, in order to solve the unstable of the carriage servo motor, the method that the aforementioned eccentric noise is cut off by using an equalizer may be proposed. However, it is difficult to efficiently solve the unstable of the carriage servo motor by this method as follows.
Namely, the aforementioned eccentric noise is generated by the eccentricity of the CD. Therefore, the frequency of the sine curve of the eccentric noise corresponds to the rotation number of the CD. The rotation of the CD is determined to about 200 rpm! to 500 rpm!, so that the frequency of the eccentric noise is about 3 Hz! to 8 Hz!.
Thus, in order to lower the level of the eccentric noise whose frequency is in the range of about 3 Hz! to 8 Hz!, an equalizer having the frequency characteristic as shown in FIG. 10 is needed. Namely, in order to remove the eccentric noise from the pulse signal PS, the equalizer must have the frequency characteristic based on the qualifications mentioned below.
(1) Obtaining the driving voltage to drive the carriage servo motor in a condition that the lens offset is about 60 .mu.m! in the direct current range, i.e., in the frequency range of not more than a frequency f1.
(2) Lowering a gain G1 in the range of the frequency f1 (about 60 Hz!) to a frequency f2 (about 1 Hz!) in order to remove the eccentric noise.
(3) Keeping the gain G1 more than predetermined level, in order to prevent an oscillation of the electric circuit of the carriage servo motor.
As mentioned above, the qualifications (2) and (3) are conflicted with each other. Because of the conflict between the qualifications (2) and (3), it is difficult to make the equalizer such that both of qualifications (2) and (3) can be sufficient. Further, in order to satisfy the qualification (3), the gain G1 cannot be drastically lowered. Therefore, the eccentric noise cannot be completely removed.